Coaxial cables commonly used today for transmission of RF signals, such as television signals, are typically constructed of a metallic inner conductor and a metallic sheath “coaxially” surrounding the core and serving as an outer conductor. A dielectric material surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In some types of coaxial cables, air is used as the dielectric material, and electrically insulating spacers are provided at spaced locations throughout the length of the cable for holding the inner conductor coaxially within the surrounding sheath. In other known coaxial cable constructions, an expanded foamed plastic dielectric surrounds the inner conductor and fills the spaces between the inner conductor and the surrounding metallic sheath.
Precoat layers are an integral part of most of these coaxial cable designs. The precoat is a thin, solid or foamed polymer layer that is extruded or applied in liquid emulsions over the surface of the inner conductor of the coaxial cable prior to the application of subsequent expanded foam or solid dielectric insulation layers. Precoats are usually made up of one or more of the following materials: a polyolefin, a polyolefin copolymer adhesive, an anti-corrosion additive and fillers. The precoat layer serves one or more of the following purposes: (1) It allows for a more controlled surface to be prepared on which to deposit subsequent extruded dielectric insulation layers. (2) It is used with or without added adhesive components to promote adhesion of the dielectric material to the center conductor in order to reduce movement of the center conductor in relation to the surrounding insulation. Significant movement of this type can cause the center conductor to pull back out of the grip of a field connector creating an open electrical circuit. This phenomenon creates a field failure commonly known as a center conductor “suck out”. (3) It is used with or without added adhesive components to promote adhesion of the precoat layer and subsequent dielectric insulation layers to prevent dielectric shrink back. (4) It is used to reduce or eliminate water migration paths at the dielectric/center conductor interface. Water migration into the dielectric of the coaxial cable has obvious detrimental impacts such as increases in RF attenuation.
Unfortunately, a consequence of the design of currently available precoats meeting the above criteria is that the precoat layer requires extra steps to remove it from the center conductor prior to installation of the connector. During field installation of the coaxial cable, the ends of the cable must be prepared for receiving a connector that joins the cable to another cable or to a piece of network electrical equipment, such as an amplifier. The preparation of the cable end is typically performed using a commercially available coring tool sized to the diameter of the cable. For coaxial cables having a foam dielectric, the coring tool has an auger-like bit that drills out a portion of the foam dielectric to leave the inner conductor and outer conductor exposed. After this “coring” step and just prior to the installation of the connector, it has been necessary for the installer to physically remove the precoat layer that remains adhered to the inner conductor. The prescribed method employs a tool with a nonmetallic “blade” or scraper that the technician uses to scrape or peel back the precoat layer, removing it from the conductive metal surface of the inner conductor.
According to the procedures prescribed in the field installation manual “Broadband Applications and Construction Manual”, sections 9.1 and 9.2 published by coaxial cable manufacturer CommScope, Inc., the field technician is instructed to use a non-metallic tool to clean the center (inner) conductor by scoring the coating on the center conductor at the shield and scraping it toward the end of the conductor. The conductor is considered to be properly cleaned if the copper is bright and shiny. If this step is not properly performed or if this step is completed with incorrect tools, such as knives or torches, the inner conductor or other components can be damaged, reducing the electrical and/or mechanical performance of the cable and reliability of the network.
From the foregoing, it should be evident that the need exists for a coaxial cable in which the center conductor precoat layer can be more easily removed from the center conductor, preferably during the coring step, when preparing the cable for receiving a standard connector.